The present disclosure relates to a digital pixel imager and, in particular, a system that compensates for motion between optics and the focal plane array, including a per-pixel bias control.
In legacy analog imagers, particularly infrared imagers, photo-current from a detector diode is integrated by a well or integration capacitor coupled to the detector diode, and then once per video frame, the voltage or charge of the well capacitor is transferred to a down-stream analog-to-digital converter (ADC), where the voltage is converted to a binary value.
One type of in-pixel ADC circuit utilizes an injection transistor such as a direct injection (DI) transistor or a buffered direct injection (BDI) transistor. In such circuits, charge from a photo-diode is accumulated on an integration capacitor during an integration period. When the end of the period is reached, the charge stored in the integration capacitor is provided to a readout circuit. Such circuits can either integrate then read or read while integrating circuits.
Line-of-sight jitter on an analog imager causes blur or effective reduction in modulation transfer function (MTF) performance. Mechanical rigidity between the optics and a focal plane array (FPA) is necessary but may not be sufficient to prevent relative motion. As such, in some cases, a relative movement between the optics and the FPA can lead to scene projection changes. For example, vibration of a vehicle, aircraft, or other platform carrying the imager can lead to scene projection changes on the FPA within the single-frame integration period. Because analog imagers average the irradiance on each individual pixel over the integration period for each frame, the scene projected on the FPA during the integration period can be affected. As a result, jitter (or scene projection changes) can only be compensated on a frame-by-frame basis, limited to vibration frequencies <1/(2*integration time) for analog readouts.